Adenovirus vectors (Ad) are the second largest group of viral vectors extensively used in clinical trials in the US. The interest in Ad has recently expanded due to its potential as a vector for vaccination against anthrax and other life threatening infection agents. Despite significant knowledge regarding Ad interactions with cells in vitro, the molecular mechanisms governing infectivity, bio-distribution and toxicity of systemically applied Ad remain poorly understood. Numerous studies of Ad pharmacokinetics in vivo have shown that within the first few minutes after systemic application, more than 90% of the virus is cleared from the circulation by the liver, and that the liver is the predominant organ in the body transduced with Ad after systemic application. Our recent data suggest, however, that liver-mediated virus clearance occurs via two distinct molecular mechanisms. The first mechanism involves fiber-dependent receptor-mediated interactions of Ad with liver cells. The second mechanism, responsible for clearance of the bulk of systemically applied virus, does not depend on fiber-hepatic cell interactions. We hypothesized that hexon, the major Ad structural protein, is the main determinant mediating virus trapping in liver tissue independently of fiber-cellular receptor interactions. The overall goal of this study is to further our understanding of molecular mechanisms underlying liver-mediated Ad clearance from the blood, and to construct a safe capsid-modified vector, efficiently transducing target cells after systemic application at lower administered doses. The specific aims of the current proposal are: 1. To evaluate the role of Ad hexon in virus clearance from the blood using wild type Ads of different serotypes; 2. To develop Ad5-based vectors with mutated hexons and analyze their bio-distribution and persistence in circulation in a mouse model; and 3. To analyze efficacy of tumor cell targeting and systemic toxicity of hexon-mutated Ad5-based vectors upon their intravascular administration in a mouse model. These studies will dramatically improve our understanding of the mechanisms governing Ad-host interactions in vivo and may ultimately lead to the development of safe and efficient Ad vectors for the therapy of a wide range of inborn and acquired human diseases. [unreadable] [unreadable] [unreadable]